This invention is in the field of laser-testing interferometers. In the manufacture of solid laser rods (specifically, Nd: YAG), it has been found that about half of the material from which the rods is made is unusable because of inclusions in the material. Visual inspection techniques weed out gross inclusions, but obviously do not detect microscopic ones. After a rod is cut from a section of crystal boule with no visible inclusions, it is tested with an interferometer for internal strain, end face flatness, and perpendicularity to the crystal axis. Unfortunately, this test does not detect microscopic inclusions; these inclusions cause scattering in the forward laser beam direction. The instant invention is a modification or improvement to an interferometer whereby these inclusions may be detected.